There has been disclosed an image forming apparatus having an abnormal discharge detection circuit for detecting abnormal discharge of corona chargers configured to charge photosensitive members. The corona chargers of the related-art image forming apparatus are disposed so as to face each of the photosensitive members, respectively. In each of the corona chargers, if a wire voltage is applied from a charging-voltage generating circuit to a charging wire, corona discharge is caused between the charging wire and a corresponding photosensitive member, whereby the surface of the corresponding photosensitive member is uniformly charged positively. The corona chargers are connected to one abnormal discharge detection circuit. The abnormal discharge detection circuit outputs an abnormal discharge detection signal to a CPU when the abnormal discharge detection circuit detects abnormal discharge occurring in any one of the corona chargers. Here, the term “abnormal discharge” includes, for example, spark discharge and arc discharge, which are different from corona discharge, caused by contamination of charging wires or the like.
In the above-mentioned image forming apparatus, the abnormal discharge detection circuit is common to the plural corona chargers, and one common signal path of the abnormal discharge detection signal to the CPU is used. Therefore, it is possible to reduce the number of input ports required for the CPU to receive the abnormal discharge detection signal. However, for example, in a printer capable of color printing, if a charging-voltage generating circuit of a corona charger corresponding to black and a charging-voltage generating circuit of corona chargers corresponding to the other colors (cyan, magenta, and yellow) are configured as separate circuits, the aspect of abnormal discharge changes. More specifically, the case where one corona charger is connected to one charging-voltage generating circuit and the case where plural corona chargers are connected to one charging-voltage generating circuit are different in outputs required for the charging-voltage generating circuit. Therefore, for example, variations of current values of abnormal discharge currents generated in the individual corona chargers are different between the above cases. Consequently, it may be impossible to accurately detect abnormal discharge occurring in each corona charger when the abnormal discharge detection circuit is configured as a common circuit between the plural corona chargers.